Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers so as to define a p-n junction. When a bias is applied across the p-n junction, holes and electrons are injected into the active layer where they recombine to generate light in a process called injection electroluminescence. Light may be emitted from the active layer through all surfaces of the LED.
As most LEDs are nearly monochromatic light sources that appear to emit light having a single color, light emitting devices or lamps including multiple LEDs that can emit light of different colors have been employed to produce white light. In these devices, the different colors of light emitted by the individual LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white.
As an alternative to combining individual LEDs to produce light emitting devices having a particular light emission spectrum, luminescent materials, or phosphors, may be used to control the color of light emitted from LEDs. A phosphor may absorb a portion of the light emitted from an LED at a given wavelength and re-emit the light at different wavelength via the principle of photoluminescence. The conversion of light having a shorter wavelength (or higher frequency) to light having a longer wavelength (or lower frequency) may be referred to as down conversion. For example, a down-converting phosphor may be combined with a blue LED to convert some of the blue wavelengths to yellow wavelengths in order to generate white light.
A widely used phosphor for white light generation is yttrium aluminum garnet (YAG), which may be doped with cerium (Ce), e.g., Y3-xCexAl5O12 or YAG:Ce. This yellow phosphor may be used in combination with a blue LED to produce white light. Compared to other phosphors based on silicates and sulfides, for example, YAG:Ce has a relatively high absorption efficiency of blue excitation radiation, a high quantum efficiency (greater than 90%), good stability in high temperature and/or high humidity environments, and a broad emission spectrum.
In some cases, a red phosphor is added to an LED component including a blue LED and a yellow or green phosphor in order to further shift the emitted light into the desired neutral white color bins on the 1931 CIE chromaticity diagram. The red phosphors most commonly used in LED components have excitation spectra that extend from less than 400 nm to about 600 nm, as shown in FIG. 1. The blue LEDs used in these components may emit blue light over a wavelength range of from 425 nm to 475 nm, or more typically from 430 nm to 470 nm, and yellow/green phosphors also present in the components may emit light over a wavelength range of 500 nm to 600 nm. Thus, a red phosphor may be excited by light from the blue LED (as intended), and also from the yellow/green phosphor. Because the light conversion efficiencies of phosphors are not 100%, light emitted from a blue LED that is down-converted by a yellow/green phosphor before reaching the red phosphor is converted less efficiently than light that reaches the red phosphor directly from the blue LED.